A known electric power steering device reduces the steering force by a steering wheel by providing the assisting force to a steering mechanism connected to a steering shaft by a motor. With the known electric power steering device, the steering wheel rotates within a predetermined limited rotation number or greater than a single rotation in the clockwise direction and in the counterclockwise direction respectively. A position of the steering wheel at which the vehicle moves straight is determined as a neutral position. Thus, a steering angle is obtained by detecting the absolute position of the steering wheel, i.e., by detecting the rotation angle of the steering wheel relative to the neutral position.
A known absolute position detection device described in Japanese Patent Laid-Open Publication No. 2003-75109 is disclosed as the sensor for detecting the steering angle by the steering wheel. With the known absolute position detection device described in Japanese Patent Laid-Open Publication No. 2003-75109, the number of pole pairs of a second resolver serving as a part of a torque sensor for detecting the steering torque of the steering wheel and the number of pole pairs of a motor resolver for detecting a motor rotation angle of an assisting motor are determined different from each other. With this construction, the absolute rotational position of the steering wheel is detected by using the characteristics that a difference of detection signal waveforms generated by a cycle difference of detection signals detected from the second resolver and the motor resolver assumes a predetermined amount based on a speed ratio between the steering wheel and the assisting motor.
Notwithstanding, with the known absolute position detection device described in Japanese Patent Laid-Open Publication No. 2003-75109, the speed ratio between the steering wheel and the assisting motor is determined based on a predetermined proportional stroke S and a predetermined lead L serving as a design value, or the like. In this case, the proportional stroke S corresponds to a moving amount of a rack shaft of a rack and pinion mechanism when the steering wheel is rotated by a single rotation. The lead L corresponds to the moving amount of the rack shaft when the assisting motor is rotated by a single rotation. The speed ratio is calculated by S/L.
Accordingly, in case, for example, a pinion gear and a rack groove, or the like, included in the rack and pinion gear mechanism have the machining error and the dispersion, or the like, the error is generated at the speed ratio determined by the predetermined design value, or the like. Further, the error is generated at the quantitative difference of the detection signal waveforms. Thus, it becomes difficult to accurately detect the absolute rotational position of the steering wheel.
A need thus exists for the present invention to provide a setting method of a control parameter and a setting device of the control parameter which enables to determine the control parameter for accurately detecting the absolute rotational position of the steering wheel at an electric power steering device. A need further exists for the present invention to provide an electric power steering device which accurately detects the absolute rotational position of the steering wheel and controls a motor for assisting the steering based on the absolute rotational position.